Widely graded loose soils (WGLS), a loose mixture comprising coarse and fine particles, are widely distributed in the western mountainous areas of China. Owing to rainfall infiltration, fine particles are eroded and migrated through the skeleton formed by coarse particles, thus altering the hydraulic and mechanical characteristics of WGLS. This paper intends to uncover the internal erosion process and its influencing factors in WGLS due to rainfall infiltration. To this end, a novel fixed-wall permeameter is developed, capable of applying an inflow rate-controlled condition and collecting effluent flowing out of the specimen. A simplified and cost-effective testing protocol is proposed for separating eroded silty clay particles, sandy gravel particles, and seepage water from the collected effluent. Several seepage tests are conducted on remolded specimens with different inflow rates and initial porosity utilizing the newly developed experimental setup and testing protocol. The results indicate four successive erosion stages in the internal erosion process: the suffusion stage, the suffosion stage, the piping stage, and the stabilization stage. Six critical hydraulic gradients correspond to the onset of the various stages. The inflow rate does not affect the critical hydraulic gradient for suffusion. However, it significantly impacts the critical hydraulic gradient for suffosion and subsequent internal erosion behavior. Increasing initial porosity does not necessarily result in higher erosion potential. These results are conducive to further understanding the formation mechanism of internal erosion-induced geologic hazards.

Soil–rock mixtures are widely encountered in geotechnical engineering projects. The instability and failure mechanism of grap-graded soil–rock mixtures under rainfall conditions has always been the focus of geological disaster research. To deeply explore the mechanism of seepage deformation of soil–rock mixtures, an indoor physical permeability test that considers soil–rock mixtures with different fine contents was conducted, and a particle-scale numerical simulation test of the permeability evolution was carried out using the coupling model of PFC3D and ABAQUS. The test results showed that the spatial distribution of fine particle loss along the height direction could be divided into three areas: top loss, middle uniform, and bottom loss area. The “island” effect of coarse particles, which is caused by excessive fine content and makes the fine particles bear more load, was eliminated with the loss of fine particles. In this preset working condition of coarse and fine particle diameters, setting FC to 35% may be the best way to fill the voids between the coarse particles. Particle migration leads to a change in the load-bearing skeleton structure, thereby causing seepage deformation. Therefore, the particle-scale numerical test method can better reproduce the seepage deformation process of grap-graded soil–rock mixtures.